U-Boot uses "ethernet0", not "eth0".

While at it, fix nearby whitespace errors (TAB instead of space before
equal sign).

Fixes: 93be50c7 ("arm64: dts: renesas: Add R-Car S4 Starter Kit support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/2f62c29e4222387a95ebadc65ba90a0cdea9b78c.1712647914.git.geert+renesas@glider.be